Hemopoiesis involves the active process of proliferation and differentation of pluripotent progenitor cells into all types of mature blood cells and some specialized tissue calls. Production of functional blood cells is regulated by specific proteins, the hemopoietic growth factors (HGFs). Some of the HGFs control maturation of a specific maturation lineage, whereas others stimulate proliferation and differentiation of progenitors along multiple pathways. Much of our knowledge of the hemopoietic differentiation process has been obtained from mouse studies in vitro and in vivo, using purified growth factors. The murine growth factor interleukin-3 (mIL-3), also termed Multi-CSF, mast cell growth factor, stem cell activating factor or several other designations, stimulates the proliferation of developmentally early, multipotent cells (CFU-S) as detected by the spleen colony assay, resulting in the production of progenitor cells along the erythroid, megacaryocyte, granulocyte/macrophage, osteoblast and several other lineages. Furthermore, mIL-3 has been implicated in replication of pluripotent stem cells, probably in synergism with other HGFs.
In recent years, several groups have succeeded in cloning mIL-3 cDNA. No results have been reported so far of identifying homologous sequences in human DNA using mIL-3 DNA as a probe. Presumably, the human gene has diverged extensively from the mIL-3 gene or has lost its function during primate evolution. However, human leukocytes were found to produce a HGF(s) which can replace mIL-3 in supporting the proliferation of murine CFU-S. Thus, the existence of a human HGF was postulated, which shares biological properties with mIL-3 and therefore could be the human homolog. Yang, Y-C, et al, Cell (1986) 47:3-10, dated 10 October discloses cDNA encoding a protein having IL-3 Like activity from gibbon T-cells, and retrieval of a genomic DNA which encodes the human counterpart. The sequence of a cDNA encoding human IL-3 can be deduced from the human gene sequence published by Yang et al. However, said article does neither disclose nor teach a method for isolation of a cDNA encoding human IL-3, nor was the production of hIL-3 achieved. This invention describes for the first the isolation of a cDNA comprising the entire coding sequence for human IL-3.
Human IL-3 protein has never been prepared in purified form, nor have its characteristics, other than its activity in certain in vitro proliferation assays and deduced primary structure, been disclosed. The present invention permits the recovery of purified human IL-3, and identification of its characteristics through recombinant production: from a cDNA clone.